1. Field of Invention
The present invention relates generally to protection switching in optical networks. More particularly, the present invention relates to allowing for a transparent switchover from a primary interface to a backup interface with respect to a multi-router automatic protection switching (APS) in a single chassis associated with an optical network.
2. Description of the Related Art
Automatic protection switching (APS) is a mechanism that is used for network survivability on synchronous optical transport network (SONET) networks in the event of a failure on a network element or a link. APS provides for a failover of traffic from a working link to a dedicated protection link when there is a failure of the working link. Multiplexed switching protection (MSP) is a substantially equivalent mechanism to APS, and is used in synchronous digital hierarchy (SDH) networks. APS schemes generally reserve a protection or backup channel with the same capacity as a working or primary channel that is to be protected. By way of example, APS uses a primary interface to carry traffic and a backup interface to switch over to as a standby interface in the event of failures associated with the primary interface.
Typically, APS is a feature that is offered in high-end routers, and may be used by customers to provide circuit-level protection for expensive, high bandwidth circuits. APS may be implemented using multiple routers on different chassis, or using multiple routers on a single chassis. FIG. 1A is a block diagram representation of a system in which multiple routers on different chassis are used to implement an APS topology. In a system 100, a first router 104a is associated with a primary interface 106a. A backup interface 106b of a second router 104b is arranged to serve as a standby to primary interface 106a. An add-drop multiplexer (ADM) 110 receives traffic on both primary interface 106a and backup interface 106b. In general, during protection switching, ADM 110 signals back and forth with both first router 104a and second router 104b using K1 and K2 line overhead bytes in SONET frames, or equivalent line overhead bytes of SDH frames, to indicate the nature and priority of a switch request. Utilizing values stored in K1 and K2 bytes in line overhead, ADM 110 determines whether to use the traffic received on primary interface 106a or the traffic received on backup interface 106b. As will be appreciated by those skilled in the art, the default condition when neither primary interface 106a nor backup interface 106b has failed is to use traffic obtained from primary interface 106a. ADM 110 forwards traffic obtained on one interface 106a, 106b, and discards any traffic obtained on the other interface 106a, 106b. 
ADM 110 forwards traffic received on one of interface 106a, 106b through a SONET network 114 towards an intended destination. Network 114, though described as being a SONET network, may instead be an SDH network. The traffic is provided network 114 to an ADM 118 associated with a destination, or remove, router 122.
Traffic originating at router 122 may be sent via ADM 118, through SONET network 114, to ADM 110 and, hence, to primary interface 106a and backup interface 106b. In general, any traffic received by ADM 110 from network 114 is forwarded from ADM 110 on both primary interface 106a and backup interface 106b. 
Having first router 104a and second router 104b on separate chassis provides chassis-level protection in the event of a failure of an overall chassis. However, multiple routers may also be implemented on a shared chassis. FIG. 1B is a block diagram representation of a system in which multiple routers, e.g., a router implementing a primary interface and a router implementing a backup interface, on a singlechassis are used to implement an APS topology. In a system 150, a router arrangement 154 that has an APS topology includes both a primary interface 156a and a backup interface 156b that are in communication with an ADM 160. ADM 160, using K1 and K2 bytes of packets contained in traffic received on primary interface 156a and backup interface 156b, identifies which interface 156a, 156b is suitable for use in obtaining traffic to be forwarded through a SONET network 164 to an ADM 168 and a remote router 172.
Traffic that is received by ADM 160 from SONET network 164 is placed by ADM 160 on both primary interface 156a and backup interface 156b. Router arrangement 154 then determines whether to use the traffic obtained via primary interface 156a or the traffic obtained via backup interface 156b. 
In either system 100 of FIG. 1A or system 150 of FIG. 1B, when a primary interface fails, a switch is made to an associated backup interface. Such a switch may be indicated in K1 and K2 bytes to provide components of a system, as for example ADMs, with information that indicates that a backup interface is to be used to receive and to forward traffic. Using information stored in K1 and K2 bytes, an ADM is informed that a switch from a primary interface to a backup interface should be made. The amount of time within which the switch is expected to occur is typically specified by standards as a time required to detect and to perform a protection switch.
When a circuit level switch is made from a primary interface to a backup interface, the switch is specified as having to occur within an approximately 50 millisecond (ms) time interval. However, such a switch typically does not include actually switching traffic from the primary interface to the backup interface. In other words, a switchover may effectively occur relative to layer 1 of an Open Systems Interconnection (OSI) reference model standard within approximately 50 ms, but a switchover that occurs relative to layer 3 of the OSI reference model standard typically takes much longer than 50 ms. That is, the layer 3 switchover, or a routing or forwarding convergence, takes longer than 50 ms and is not included in SONET specification for a 50 ms switchover timeframe.
A device which implements a routing protocol generally resets the routing protocol when it is determined that a primary interface is no longer being used. As will be appreciated by those skilled in the art, a routing protocol is the language a router “speaks” with other routers in order to share information about the reachability and status of a network. When a routing protocol is reset in the event of a topology change, e.g., in the event of a determination that a primary interface is no longer usable, a fairly significant amount of time may elapse as a result of a resetting or reconverging process. As a result of a routing protocol being reset, traffic may often be dropped or otherwise lost during a switchover relative to layer 3.
With reference to FIG. 2, one method is shown of switching traffic from using a path associated with a primary interface to a different path, e.g., a path associated with a backup interface. A process 200 of switching from a primary path to a backup path begins at step 204 in which a primary interface fails. It should be appreciated that a failure of a primary interface may generally be a failure that has an affect on the primary interface. Once the primary interface fails, a switchover from the primary interface to an associated backup interface is performed in step 208. Such a switchover generally involves a circuit level switch from the primary interface to the backup interface.
After the circuit level switchover is performed, a process that embodies a routing protocol identifies the switchover to the backup interface in step 212. The routing protocol, which may be an interior gateway protocol (IGP) such as an enhanced interior gateway routing protocol (EIGRP) embodied as a process, identifies the switchover when the routing protocol determines that it is no longer communicating using the primary interface. IGP is a protocol for exchanging routing information between gateways within an autonomous network that may be used by internet protocol (IP) or other network protocols to specify how to route traffic. Open shortest path first (OSPF) and IS-IS are examples of an IGP. EIGRP, or a process that embodies EIGRP, notifies nodes within a network of topology changes in a system by transmitting substantially only the change, as opposed to an entire topology table. Each router using EIGRP is aware of the state of each of its neighbors, notifies each neighbor of changes in topology. EIGRP typically uses a diffusing-update algorithm (DUAL) to identify whether a path is looped or loop-free, and to identify a most efficient route to a particular destination.
The routing protocol effectively resets itself in step 216 after the switchover is identified in step 212. Resetting a routing protocol generally involves reconverging a network in the event of a topology change such as the failure of a primary interface. From step 216, process flow moves to step 220 in which traffic is switched to a new route identified by the routing protocol. As previously mentioned, switching traffic to a new route typically results in a loss of some traffic because of the downtime in a network associated with the resetting or reconverging process. Once the traffic is switched, the process of switching traffic from a primary interface is completed.
The loss of some packets contained in traffic when a process that implements a routing protocol reconverges a network prior to a traffic level switchover is often considered to be unacceptable, particularly when the network is used to transmit data traffic. Even though a circuit level switchover from a primary interface to a backup interface may be relatively fast, e.g., may occur in less than approximately 50 ms, from a layer 3 perspective, the traffic level switchover occurs much more slowly due to downtime associated with reconverging the network and causes some traffic to be lost. The downtime and the loss of traffic may be highly detrimental, e.g., loss of traffic associated with applications is highly undesirable.
Therefore, what is needed is a method and an apparatus for efficiently switching traffic from a primary interface to a backup interface in a multi-router APS implementation, without a significant loss of traffic when the primary interface fails. That is, what is desired is a system which allows for a transparent switchover to occur relative to layer 3 in a system which utilizes multi-router APS implemented on a shared chassis.